1. Field of the Invention
The instant invention relates to a new magneto-optically readable recording medium and a new method of thermomagnetic recording. More particularly, the instant invention relates to a new magneto-optically readable recording medium consisting of a substrate and a first and second layer, said layers possessing perpendicular magnetic anisotropy and can be easily magnetized perpendicular to the layer plane, said first layer possessing higher coercive force and lower Curie point relative to said second layer, said second layer comprised essentially of Gd alloys, and said first and second layers exchange coupling with each other. Further the instant invention relates to a new method of thermomagnetic recording employing said magneto-optically readable recording medium consisting of performing writing of information by heating said medium at approximate Curie point of said higher coercive force layer as the first layer by employing converged laser light.
2. Description of the Prior Art
Hitherto, magneto-optically readable recording media for been provided, which consist of a substrate made of non-magnetic material, such as glass, ceramics, plastics and the like, and a recording layer made of a metallic magnetic material, such as an alloy comprising two or more metals e.g. iron, cobalt, chromium, manganese, gadolinium, terbium, bismuth and the like, or metallic oxide e.g. iron oxide, rare-earth metal-iron garnet and the like. In said magneto-optically readable recording media laser light is applied to the surface of the recording layer to heat the respective applied part of the surface at approximately its Curie point or to more than the compensation point and a bias magnetic field is applied to said heated part to cause the magnetization of said heated part to become oriented in the opposite direction to another part. In this way the writing of information is performed. In addition, another linear polarized laser light is applied to the surface of the recording layer in which information is recorded and the reflecting light from the surface is optically detected by using the photo-detector. Thus, the readout of recorded information is performed.
The recording layer of the above-mentioned magneto-optically readable recording medium consists of a layer of a metallic magnetic material. Gadolinium alloys (Gd alloys) may be preferable as said metallic magnetic material since Gd alloys give said recording medium an excellent SN ratio. Gd alloys, however, possess one disadvantage, namely said recording medium consisting of Gd alloys has low coercive force and therefore the information written in the recording medium is unstable.
Accordingly, an object of the instant invention is to improve said fault of the recording layer consisting of Gd alloys.
Said object of the instant invention can be attained by exchange-coupling a metallic magnetic material layer having higher coercive force with the Gd alloys layer.
1. In case of writing at approximately the Curie point, the material having a lower Curie point must be used as the recording layer since it is difficult to heat the recording layer at high temperature when the laser is miniaturized to reduce the writing power, or when the writing speed (Bit Rate) is increased. However in case of low Curie point materials, the signal of the readout may be low since the oriented magnetization is apt to become disoriented even at usual temperature.
Further, the intensity of light must be limited to lower than a certain value. This is because the recorded information is erased by a significant increase of the temperature of the applied recording medium surface when the intensity of light applied to the surface of the recording layer is increased during readout. Consequently the S/N ratio (Signal/Noise Ratio) may be small when such small intensity of light is used since the S/N ratio is proportional to the square root of the intensity of light. The S/N ratio may be reduced by above-mentioned two factors.
2. In case of writing at more than the approximate compensation point, a large margin in the composition of the recording medium is not permissible since the range of the composition of the recording medium must be limited to obtain a high coercive force. This in turn requires strictness regarding the composition of the recording medium and causes trouble in the preparation process thereof. Further, the recorded information is apt to be affected by the external magnetic field and temperature; and the recording density is also reduced.